The components of internal combustion engines are routinely subjected to harsh conditions during operation. The combustion of fuels leads to high temperatures, high pressures and a corrosive environment for many components, not least inside the combustion chamber. At the same time, the movement of reciprocating parts leads to wear and frictional losses in the engine. Heavy duty vehicle engines have, due to their size, typically been provided with loose cast iron liners. Also other vehicle components such as gears or synchronizers are subject to frictional wear.
Vehicle component parts such as cylinder liners can be coated using any of a number of methods, depending on the coating required, the coating thickness and the conditions that the component tolerates. Thermal spray coating methods are among the most commonly used.
Thermal spray coating involves propelling melted or molten spray material at high speed onto a cleaned and prepared component surface. Methods of thermal spray coating can be further subdivided and include flame spray coating, electric arc spray coating, high velocity oxy-fuel spray coating and plasma spray coating. All thermal spray methods are characterized by the high temperatures that the thermal-coating compositions are subjected to during the coating process, which can be from approximately 2600° C. to 16000° C. depending on the method used.
An existing production process for coating cylinder liners is the plasma spray coating of grey cast iron liners with a powder comprising stainless steel and alumina-zirconia composite. Such coatings greatly improve the cylinder properties with regards to wear and corrosion. Honing of the coated surface also provides good lubrication properties of the surfaces that liquid lubricants can reach. However, due to increasing demands for better fuel economy and lower emissions, there remains a desire to further reduce friction in the engine, especially at surfaces that may be poorly lubricated using liquid lubricant systems.
The component parts of a combustion engine are typically lubricated using an oil-based lubricant system. While such systems have been proven to be effective, sufficient lubrication is difficult to achieve in all operating conditions and at all areas of frictional contact. This is especially a problem for vehicles implementing stop/start drive systems, for example hybrid vehicles. Therefore, the use of solid lubricants is attracting interest.
Solid lubricants are materials which despite being in the solid phase are able to reduce friction between two surfaces sliding against each other without the need for a liquid lubricant medium. Common solid lubricants include graphite, molybdenum disulphide and boron nitride. Solid lubricants can be used to reduce friction in situations where the use of conventional liquid lubricants is insufficient or inappropriate. Such uses include interfaces in reciprocating engines and turbines, such as in the cylinder liners of an internal combustion engine.
A number of methods for the thermal spraying of solid lubricant coatings are known in the art.
In U.S. Pat. No. 5,332,422A a plasma sprayable powder for coating surfaces such as cylinder bores of an internal combustion engine is disclosed. The plasma sprayable powder comprises grain size agglomerates of i) a plurality of solid lubricant particles; ii) fusable ingredients adjacent the solid lubricant particles; and iii) a low melting medium binding the solid lubricant particles and fusible particles together in agglomerated grains.
In EP1785503A1 a coating with a low coefficient of friction that is wear resistant, corrosion resistant and heat resistant is disclosed. The coating is prepared by a process comprising the steps of: providing hard face material particles; agglomerating the hard face material particles using a first binder material; providing solid lubricant material particles; agglomerating the solid lubricant material particles using a second binder material; and applying to a substrate the agglomerated hard face material particles and the agglomerated solid lubricant material particles via a thermal spray process.
In US2008/0145554A1 a method of making a thermal spray powder is disclosed. The method comprises: providing a powder comprising a plurality of porous particles; infiltrating a mixture comprising a solvent and a plurality of solid lubricant particles into the porous particles; and heat-treating the powder to a temperature sufficient to evaporate the solvent.
In WO2011/094222A1 a thermal spray powder is disclosed. The thermal spray powder comprises a solid lubricant clad with at least one of a metal and metal alloy, mechanically blended with a metal or polymer clad with at least one of a number of specified metals or metal alloys.
In U.S. Pat. No. 5,766,690A a method of producing a self-lubricating coating on a substrate is disclosed. The method comprises the steps of: providing a matrix of particles including chromium carbide; mixing with the matrix a solid lubricant including barium fluoride and calcium fluoride particles to form a composite material; providing a high velocity oxy-fuel gas stream; and introducing the composite material into the gas stream to spray deposit the composite material onto the substrate.
There are a number of problems associated with these known methods. Many are based upon the principle of “encapsulating” or agglomerating the solid lubricant in order to avoid volatilization or decomposition. Manufacturing of such coating materials is complicated and expensive. If such an “encapsulated” solid lubricant is not used, the high temperatures used in thermal spray methods tend to volatilize or decompose the solid lubricant. This results in low incorporation of the solid lubricant into the final coating, manufacturing difficulties, production down-time and the loss of material since the final coating contains relatively little solid lubricant in relation to the amount used in the powder mixture. Moreover, the volatilized material risks being undesirably deposited on proximate surfaces if sufficient fume extraction is not obtained. Attempting to rectify these problems by reducing the temperature of the spray results in lower deposition efficiency of the other, less-volatile, constituents, such as ceramic particles and metal alloy particles. Using larger solid lubricant particles sizes or agglomerating the coating mixture can to some extent mitigate these problems, but instead leads to decreased anti-friction effects or a complex manufacturing procedure for the coating mixture.
There therefore exists a need for a method for producing a coating comprising solid lubricant that reduces or eliminates the problems associated with the methods known in the art.